Refrigeration cycle apparatus

ABSTRACT

In a refrigeration cycle apparatus that recovers power in an expander, obtaining a refrigeration cycle apparatus that is capable of reliably starting up the expander compared to conventional refrigeration cycle apparatuses. The refrigeration cycle apparatus includes a refrigerant circuit having a first compressor, a radiator, an expander and an evaporator connected in series with a piping; and a second compressor disposed between the first compressor and the radiator, the second compressor being driven by power recovered by the expander. The second compressor being a positive displacement compressor. The refrigeration cycle apparatus, further including a pressure regulating device (a bypass and an on-off valve) that maintains a pressure on a discharge side of the second compressor to be lower than a pressure on a suction side of the second compressor at least until the second compressor is started up.

TECHNICAL FIELD

The present invention relates to a refrigeration cycle apparatusrecovering power by using an expander.

BACKGROUND ART

In conventional refrigeration cycle apparatuses used for refrigeration,air-conditioning, and the like, for example, an expansion process iscarried out in a positive displacement expander, and an expansion powerrecovered by this process is used in the compression process carried outin a positive displacement compressor.

However, since an expander and a compressor that is driven by powerrecovered by an expander are rotary machines, a “negative power” isgenerated therein by frictional resistance, mechanical loss, and thelike. Accordingly, in order to start up an expander and a compressorthat is driven by power recovered by an expander, power that canovercome this “negative power” is required. Hence, there has beenproposed a refrigeration cycle apparatus that is designed to reduce the“negative power” that hinders the start up (rotation) of the expanderand the compressor that is driven by power recovered by the expander,and a refrigeration cycle apparatus that is designed to increase the“positive power” (power that rotates the expander) at the start up ofthe expander.

A refrigeration cycle apparatus as above is proposed, for example, that“has a structure in which a drive shaft of the other compressor isconnected to an output shaft of an expansion mechanism. A structure inwhich a bypass pipe is provided that connects a gas suction port and agas discharge port and that bypasses the other compressor, the bypasspipe provided with a check valve that regulates the communication of arefrigerant from the gas discharge port to the gas suction port” (referto Patent Literature 1, for example).

Further, a refrigeration cycle apparatus as above is proposed thatincreases the power that can be recovered by the expander by increasingthe pressure difference between the inlet side and the outlet side ofthe expander (refer to Patent Literature 2, for example).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 11-94379 (paragraphs [0009] and [0013], FIG. 1)

Patent Literature 2: Japanese Unexamined Patent Application PublicationNo. 2006-132818 (paragraphs [0014] to [0021])

SUMMARY OF INVENTION Technical Problem

For example, in the refrigeration cycle apparatus described in PatentLiterature 1, a bypass pipe equalizes pressure between the dischargeside pressure and the suction side pressure of the compressor. Thisfacilitates the start up of the expander (expansion mechanism) and thecompressor that is connected to this expander with a shaft.

However, the compressor that is connected to the expander with a shaftis a positive displacement compressor, and, thus the pressure insideincreases.

FIG. 11 is an explanatory diagram of Patent Literature 1 illustrating apressure change in the compression chamber of the compressor connectedto the expander with a shaft. The pressure in the compression chamber ofthe compressor changes during the process depicted by arrows in FIG. 11.As mentioned above, the compressor is a positive displacementcompressor, and, thus, the pressure increases in the inside. Therefore,in order to start up the compressor, a compression power amounting tothe area C illustrated in FIG. 1 is needed. That is to say, even whenthe suction side and the discharge side of the compressor is bypassed,as illustrated in Patent Literature 1, a “negative power” will exist.Thus, in some cases, the “negative power” becomes larger than the“positive power” obtained in the expander, and a possibility of theexpander not starting up arises.

Further, the start up of expanders and compressors are influenced by thestatic friction acting on thrust bearings, radial bearings, and the likeof the expanders and compressors. This static friction is larger thanthe kinetic friction acting on the thrust bearings, radial bearings, andthe like while the expanders and compressors are driven. Therefore, inorder to start up expanders and compressors, a “positive power”overcoming the static friction acting on the thrust bearings, radialbearings, and the like of the expanders and compressors will also berequired, and the start up of the expanders and the compressors becomeeven more unstable.

For example, in a scroll compressor, in order to reduce the load appliedto the thrust bearing (the friction acting on the thrust bearing),refrigerant in a compression process is typically introduced into theback side of the oscillating scroll. For example, in a scroll expander,in order to reduce the load applied to the thrust bearing (the frictionacting on the thrust bearing), refrigerant in the expansion process istypically introduced into the back side of the oscillating scroll.However, these methods of reducing the load applied to the thrustbearing (the friction acting on the thrust bearing) are under theassumption that the thrust bearing is rotating. That is, the methods arefor reducing kinetic friction acting on the thrust bearing. Accordingly,it will be not possible to expect the static friction acting on thethrust bearing to be reduced while in a state in which the oscillatingscroll is suspended (a state in which the pressure to reduce the thrustload is not acting on the back side of the oscillating scroll). Ifpressure to reduce the thrust load is acting on the back side while theoscillating scroll is suspended, it is the pressure due to the leakageof the refrigerant from the expansion chamber or compression chamber. Inthese expanders and compressors, the performance improvement effectduring a steady state in which the oscillating scroll is oscillatingwill be remarkably diminished, and the primary objective (expansion andcompression of the refrigerant) will not be accomplished.

Further, if the expander or the compressor should fail to start up once,and is mechanically stuck (jammed), a driving source such as a motorwill be needed to be rotated with a torque surpassing the jamming.Alternatively, to cancel the jamming, the driving source needs to berotated slightly backwards. In any event, they are not reliable start upmethods.

Furthermore, in the refrigeration cycle apparatus of the above-mentionedPatent Literature 2, start up of the expander is facilitated byincreasing the pressure difference between the suction side and thedischarge side of the expander. However, an expander is typicallydesigned based on the steady state. That is, an expander is not designedunder the assumption that the start up of the expander will be carriedout in a state in which the pressure difference of the inlet side andthe outlet side of the expander is small.

Accordingly, when a refrigerant with high density (with isopycnic linesof low density) flows into the expander, as shown in FIG. 12, thepressure difference in the expansion chamber becomes large, and, thus,the refrigerant becomes over-expanded. Specifically, the power recoveredby the expander disadvantageously becomes a power (negative power)corresponding to “area F—area G”, and a problem that the expander isunable to continue driving occurs.

An expander designed with focus on its start up can be considered, butthen, the expansion becomes insufficient during steady operation andadequate performance improvement effect cannot be obtained, and theprimary objective cannot be achieved.

The present invention has been made to solve at least one of the aboveproblems and an object of the invention is, in a refrigeration cycleapparatus recovering power with an expander, to obtain a refrigerationcycle apparatus that is capable of reliably starting up an expandercompared to conventional refrigeration cycle apparatuses.

Solution to Problem

An refrigeration cycle apparatus according to the invention includes arefrigerant circuit including a first compressor, a first heat exchangerthat serves as a radiator or a condenser, an expander, and a second heatexchanger that serves as an evaporator connected in series with apiping; and a second compressor that is driven by power recovered by theexpander, the second compressor disposed between the first compressorand the first heat exchanger in the refrigerant circuit, in which thesecond compressor is a positive displacement compressor, therefrigeration cycle apparatus, further comprising a pressure regulatingdevice that maintains a pressure on a discharge side of the secondcompressor to be lower than a pressure on a suction side of the secondcompressor at least until the second compressor is started up.

An refrigeration cycle apparatus according to the invention includes arefrigerant circuit including a first compressor, a first heat exchangerthat serves as a radiator or a condenser, an expander, and a second heatexchanger that serves as an evaporator connected in series with apiping; and second compressor that is driven by power recovered by theexpander, the second compressor disposed between the first compressorand the first heat exchanger in the refrigerant circuit, in which thesecond compressor is a positive displacement compressor, therefrigeration cycle apparatus, further comprising a pressure regulatingdevice that maintains a pressure on a discharge side of the secondcompressor to be lower than a pressure on a suction side of the secondcompressor at least until the second compressor is started up.

An refrigeration cycle apparatus according to the invention includes arefrigerant circuit including a first compressor, a first heat exchangerthat serves as a radiator or a condenser, an expander, and a second heatexchanger that serves as an evaporator connected in series with apiping; and a second compressor that is driven by power recovered by theexpander, the second compressor disposed between the first compressorand the first heat exchanger in the refrigerant circuit, in which thesecond compressor is a positive displacement compressor, therefrigeration cycle apparatus, further comprising an expander startupfacilitating device that controls a pressure on a discharge side of theexpander to be lower than a pressure on a suction side of the expanderand that controls a density of the refrigerant flowing into the expanderat least until the second compressor is started up.

An refrigeration cycle apparatus according to the invention includes arefrigerant circuit including a first compressor, a first heat exchangerthat serves as a radiator or a condenser, an expander, and a second heatexchanger that serves as an evaporator connected in series with apiping; and a second compressor that is driven by power recovered by theexpander, the second compressor disposed between the second heatexchanger and the first compressor in the refrigerant circuit, in whichthe second compressor is a positive displacement compressor, therefrigeration cycle apparatus, further comprising an expander startupfacilitating device that controls a pressure on a discharge side of theexpander to be lower than a pressure on a suction side of the expanderand that controls a density of the refrigerant flowing into the expanderat least until the second compressor is started up.

Advantageous Effects of Invention

A refrigeration cycle apparatus according to the invention is providedwith a pressure regulating device that maintains the pressure on thedischarge side of a second compressor lower than the pressure on thesuction side of the second compressor at least until the secondcompressor is started up. Hence, the compression power is reducedcompared to conventional refrigeration cycle apparatuses, and theexpander can be reliably started up compared to conventionalrefrigeration cycle apparatuses.

Further, the refrigeration cycle apparatus according to the invention isprovided with an expander startup facilitating device that controls thedensity of the refrigerant flowing into the expander such that thepressure on the discharge side of the expander is lower than thepressure on the inlet side of the expander at least until the expanderis started up. Accordingly, even when the expander is started up in astate in which a pressure difference is small between the inlet side andthe outlet side of the expander, high-density refrigerant flowing intothe expander can be prevented. Hence, the expander can be reliablystarted up compared to conventional refrigeration cycle apparatuses.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a refrigerant circuit diagram of a refrigeration cycleapparatus of Embodiment 1.

FIG. 2 is a refrigerant circuit diagram showing a refrigerant flowduring a steady state of the refrigeration cycle apparatus of Embodiment1.

FIG. 3 is a refrigerant circuit diagram showing a refrigerant flowduring a start up of the refrigeration cycle apparatus of Embodiment 1.

FIG. 4 is an explanatory diagram illustrating a pressure change in anexpansion chamber of an expander during the start up of the expander ofEmbodiment 1.

FIG. 5 is an explanatory diagram illustrating a pressure change in acompression chamber of a second compressor during the start up of thesecond compressor of Embodiment 1.

FIG. 6 is another refrigerant circuit diagram of the refrigeration cycleapparatus of Embodiment 1 of the invention.

FIG. 6 is still another refrigerant circuit diagram of the refrigerationcycle apparatus of Embodiment 1 of the invention.

FIG. 8 is a refrigerant circuit diagram of a refrigeration cycleapparatus of Embodiment 2.

FIG. 9 is a refrigerant circuit diagram showing a refrigerant flowduring a steady state of the refrigeration cycle apparatus of Embodiment2.

FIG. 10 is a refrigerant circuit diagram showing a refrigerant flowduring a start up of the refrigeration cycle apparatus of Embodiment 2.

FIG. 11 is an explanatory diagram illustrating a pressure change in acompression chamber of a compressor connected to an expander with ashaft shown in Patent Literature 1.

FIG. 12 is an explanatory diagram illustrating a pressure change in anexpansion chamber when a high-density refrigerant flows into an expanderduring start up of the expander shown in Patent Literature 2.

DESCRIPTION OF EMBODIMENT Embodiment 1

Embodiment of the invention will be described below with reference tothe drawings.

FIG. 1 is a refrigerant circuit diagram of a refrigeration cycleapparatus of Embodiment 1 of the invention.

A refrigeration cycle apparatus 1 uses carbon dioxide as a refrigerantand includes a first compressor 2, a second compressor 3, a radiator 4,an expander 5, and an evaporator 6 connected in order with a refrigerantpiping. Further, a drive shaft of the second compressor 3 and a driveshaft of the expander 5 are connected with a shaft 7. Note that theradiator 4 and the evaporator 6 may be disposed in plural numbers.

The first compressor 2 is equipped with, for example, a motor that isdriven with supply of electrical power, and is capable of drivingindependently. The second compressor 3 is a positive displacementcompressor and is driven by power recovered by the expander 5. Theexpander 5 is a positive displacement expander and supplies the powerrecovered during the expansion of the refrigerant to the secondcompressor 3. Additionally, in the vicinity of the radiator 4, a fan 4 ais provided that sends air (heat medium), which exchanges heat with therefrigerant flowing in the radiator 4, to the radiator 4. In thevicinity of the evaporator 6, a fan 6 a is provided that sends air (heatmedium), which exchanges heat with the refrigerant flowing in theevaporator 6, to the evaporator 6.

Note that the radiator 4 corresponds to a first heat exchanger in theinvention. In addition the evaporator 6 corresponds to a second heatexchanger in the invention. Fan 4 a corresponds to a heat medium sendingdevice.

In the refrigeration cycle apparatus 1, a check valve 10 and a bypass isalso provided. The check valve 10 is disposed between the radiator 5 andthe expander 5, and regulates the refrigerant from flowing from theexpander 5 to the radiator 4. One end of the bypass 8 is connectedbetween the first compressor 2 and the second compressor 3, and theother end is connected between the check valve 10 and the expander 5.This bypass 8 is provided with an on-off valve 9 that closes and opensthe bypass 8.

Additionally, the refrigeration cycle apparatus 1 is disposed with atemperature sensor 21 on the discharge side of the second compressor 3,the temperature sensor 21 serving as a refrigerant temperature measuringdevice.

A controller 100 controls the rotation speed of the motor equipped inthe first compressor 2, rotation speed of the fan 4 a, rotation speed ofthe fan 6 a, and the closing and opening of the on-off valve 9. Thiscontroller 100 also receives the detection value of the temperaturesensor 21.

Description of Operation

Description of the operation of the refrigeration cycle apparatus 1configured as above will be made. First, the operation of therefrigeration cycle apparatus 1 during steady operation will bedescribed. Then, the operation of the refrigeration cycle apparatus 1during a start up will be described.

(Operation During Steady Operation)

The operation of the refrigeration cycle apparatus 1 during the steadyoperation will be described.

FIG. 2 is a refrigerant circuit diagram showing the refrigerant flowduring the steady state of the refrigeration cycle apparatus accordingto Embodiment 1 of the invention. During the steady state, the on-offvalve 9 is in a closed state. That is, in the steady state, therefrigerant is not allowed to flow in the bypass 8. Note that in FIG. 2,piping in which the refrigerant flows is depicted with thick lines.

The refrigerant that has been compressed into a high-temperaturemiddle-pressure refrigerant in the first compressor 2 is discharged fromthe first compressor 2. This high-temperature middle-pressurerefrigerant is compressed in the second compressor 3 into ahigh-temperature high-pressure state (supercritical state), and flowsinto the radiator 4. The refrigerant that has flowed into the radiator 4transfers heat to the air sent by the fan 4 a and turns into alow-temperature high-pressure refrigerant. This low-temperaturehigh-pressure refrigerant passes through the first check valve 10 andflows into the expander 5. The refrigerant that has flowed into theexpander 5 is decompressed into a low-pressure refrigerant with lowdryness. During this decompression process, the expander 5 recoverspower. Then, the recovered power is supplied to the second compressor 3through the shaft 7. The low-pressure refrigerant with low dryness thathas flowed out from the expander 5 flows into the evaporator 6. Therefrigerant that has flowed into the evaporator 6 receives heat from theair sent from the fan 6 a and turns into a low-pressure refrigerant withhigh dryness or a low-pressure super-heated gas refrigerant. Therefrigerant that has flowed out of the evaporator 6 is sucked into thefirst compressor 2.

Since the power recovered by the expander 5 is used as compression powerin the second compressor 3, the power required in the first compressoris reduced by the amount of power recovered. Hence, the refrigerationcycle apparatus 1 achieves energy saving.

(Operation During Start Up)

Next, the operation of the refrigeration cycle apparatus 1 during startup will be described.

FIG. 3 is a refrigerant circuit diagram showing the refrigerant flowduring the start up of the refrigeration cycle apparatus of Embodiment 1of the invention. During the start up, the on-off valve 9 is in anopened state. That is, in the start up, the refrigerant is allowed toflow in the bypass 8. Note that in FIG. 3, piping in which therefrigerant flows is depicted with thick lines.

During the start up, since the second compressor 3 is still suspended,the refrigerant that has been compressed into a high-temperaturemiddle-pressure refrigerant in the first compressor 2 flows through thebypass 8 and reaches the expander 5. At this time, the check valve 10prevents the refrigerant flowing out of the bypass 8 to flow to theradiator 4 and the discharge side of the second compressor 3.Specifically, during the state in which the second compressor 3 issuspended, the pressure on the suction side of the second compressor 3is the pressure of the refrigerant that has been discharged from thefirst compressor 2, which is higher than the pressure on the dischargeside of the second compressor 3.

Note that during the state in which the second compressor 3 issuspended, even if the check valve 10 is not provided, the pressure onthe suction side of the second compressor 3 is higher than the pressureon the discharge side of the second compressor 3. The time for thesecond compressor 3 to start up after the first compressor 2 has startedup is a few seconds (with the refrigeration cycle apparatus 1 ofEmbodiment 1, about two to three seconds, for example). Accordingly, therefrigerant flowing in the discharge side of the second compressor 3 isstored in the radiator 4 (the radiator 4 serving as a buffer), andtherefore the pressure rise on the discharge side of the secondcompressor 3 becomes slack.

That is, the bypass 8 and the on-off valve 9 are the pressure regulatingdevice of the invention. In Embodiment 1, the check valve 10 is providedin order to reliably obtain the pressure difference between the suctionside of the second compressor 3 and the pressure of the discharge sidethereof.

Further, with the start up of the first compressor 2, the refrigerant onthe outlet side of the expander 5 is sucked into the first compressor 2via the evaporator 6. Specifically, when the expander 5 is in asuspended state, the pressure on the discharge side of the expander 5becomes smaller than the inlet side of the expander 5. Further, sincethe refrigerant flowing into the inlet side of the expander 5 is arefrigerant that has not passed through the radiator 4, the refrigerantis low in density. That is, the bypass 8 and the on-off valve 9 are theexpander startup facilitating device of the invention. Note that duringthe state in which the second compressor 3 is suspended, even if thecheck valve 10 is not provided, the refrigerant flowing into the inletside of the expander 5 is a low-density refrigerant that has not passedthrough the radiator 4. Accordingly, the check valve 10 does not have tobe a constitution of the expander startup facilitating device.

When the pressure difference between the pressure on the inlet side ofthe expander 5 and the pressure on the outlet side of the expander 5(hereinafter referred to as “pressure difference of the expander 5”)becomes large, the expander 5 is started up (the driving starts).

At this time, the pressure in the expansion chamber of the expander 5 isas shown in FIG. 4.

FIG. 4 is an explanatory diagram illustrating a pressure change in anexpansion chamber of the expander during the start up of the expanderaccording to Embodiment 1 of the invention. The pressure in theexpansion chamber of the expander 5 changes during the process depictedby arrows in FIG. 4. Further, for reference purpose, the pressure changein the expansion chamber during start up of the expander according toPatent Literature 2 will be depicted with a broken line.

Since the pressure difference of the expander 5 during the start up issmaller than the pressure difference of the expander 5 during the steadystate, the refrigerant is slightly over-expanded, but power (positivepower) corresponding to “area D—area E” can be obtained. Thus, thedriving of the expander 5 can be continued.

Meanwhile, the pressure of the compression chamber of the secondcompressor 3 that is connected to the expander 5 via the shaft 7 changesas shown in FIG. 5.

FIG. 5 is an explanatory diagram illustrating a pressure change in thecompression chamber of the second compressor during the start up of thesecond compressor according to Embodiment 1 of the invention. Thepressure in the compression chamber of the second compressor 3 changesduring the process depicted by arrows in FIG. 5.

Since the pressure on the suction side of the second compressor 3 islarger than the pressure on the discharge side thereof (the pressure isinverse), it is supercompressed. The compression power at this time isthe power corresponding to the “area A—area B”, and is smaller than thatof the conventional refrigeration cycle apparatus that equalizes thepressure on the discharge side and the suction side of the compressor(Patent Literature 1, for example). Accordingly, it is easier to startup the second compressor 3 than the conventional refrigeration cycleapparatus. Further, depending on the extent of the inverse pressure, apower recovery corresponding to area B—area A can be obtained. The powerin proportion to this will contribute to the stable start up of thesecond compressor 3.

Once the expander 5 and the second compressor 3 is started up, it ispossible to continue the driving of the expander 5 and the secondcompressor 3 even when the on-off valve 9 is closed. However, inEmbodiment 1, in order to reliably continue the driving of the expander5 and the second compressor 3, the on-off valve 9 is in an opened stateuntil the refrigeration cycle apparatus 1 is capable of operating in thesteady state.

More specifically, the controller 100 controls the on-off valve 9 asbelow.

When the second compressor 3 is driven continuously, the refrigeranttemperature discharged from the second compressor 3 rises. Additionally,the pressure on the discharge side of the second compressor 3 becomeslarger or equal to the pressure on the suction side. That is, it ispossible to operate the refrigeration cycle apparatus 1 in the steadystate.

In the refrigeration cycle apparatus 1, the temperature of therefrigerant discharged from the second compressor 3 is detected with thetemperature sensor 21. In addition, the controller 100 determines thatthe refrigeration cycle apparatus 1 is capable of operating in thesteady state when the detection temperature of the temperature sensor 21is above or equal to a certain threshold value, and closes the on-offvalve 9.

Note that even if a delay occurs in determining that the refrigerationcycle apparatus 1 is capable of operating in the steady state, becausethe check valve 10 is provided, the refrigerant flows to the expander 5without the discharge pressure of the second compressor 3 risingsuddenly. Accordingly, it is possible to reliably start up therefrigeration cycle apparatus 1 without the operation of a protectivedevice for high pressure and high temperature.

As described above, in the above-configured refrigeration cycleapparatus 1, the pressure on the suction side of the second compressor 3is made to be larger than the pressure on the discharge side thereof atleast until the second compressor 3 is started up. Further, at leastuntil the expander 5 starts up, the pressure on the outlet side of theexpander 5 is made to be smaller than the pressure on the inlet side ofthe expander 5, and the refrigerant flowing to the inlet side of theexpander 5 is made to be low in density. Accordingly, it is possible tostart up the second compressor 3 and the expander 5 more reliably thanthe conventional refrigeration cycle apparatus.

Note that, it goes without saying that by merely making the pressure onthe inlet side of the second compressor 3 to be larger than the pressureon the outlet side of the second compressor 3, the second compressor 3and the expander 5 can be started up more reliably compared toconventional refrigeration cycle apparatuses. Further, it goes withoutsaying that by merely making the pressure on the outlet side of theexpander 5 to be smaller than the pressure on the inlet side of theexpander 5, the second compressor 3 and the expander 5 can be started upmore reliably compared to conventional refrigeration cycle apparatuses.

Furthermore, the invention may be embodied by providing a four-way valvein the refrigeration cycle apparatus so that the flows of therefrigerant may be switched.

FIG. 6 is another refrigerant circuit diagram of the refrigeration cycleapparatus according to Embodiment 1 of the invention. In thisrefrigeration cycle apparatus 51, a four-way valve 14 is disposed on anoutlet side of a second compressor 3. With this four-way valve 14, apassage of a refrigerant discharged from a second compressor 3 isswitched between a passage flowing to a radiator 4 and a passage flowingto an evaporator 6. Further, the four-way valve 14 switches arefrigerant passage flowing into a first compressor 2 between thepassage from the evaporator 6 and the passage from the radiator 4. Notethat when the refrigerant discharged from the second compressor 3 flowsinto the evaporator 6 (when the refrigerant flows from the radiator 4into the first compressor 2), the radiator 4 turns into an evaporator,and the evaporator 6 turns into a radiator.

Further, a four-way valve 15 is disposed on an inlet side of an expander5. The four-way valve 15 switches the refrigerant passage flowing intothe expander 5 between a passage from the radiator 4 and a passage fromthe evaporator 6.

When the above refrigeration cycle apparatus is used in anair-conditioning apparatus, the air-conditioning apparatus will becapable of carrying out both a cooling operation and a heatingoperation.

Note that since the expander 5 is of a positive displacement type, therefrigerant can only be allowed to flow in one direction. Therefore, acheck valve 10 may be provided in the vicinity of the inlet port of theexpander 5, and a bypass 8 may be provided between the check valve 10and the expander 5.

Additionally, in order to further increase the energy efficiency of therefrigeration cycle apparatus of Embodiment 1, an intercooler 22 may beprovided between the first compressor 2 and the second compressor 3.Note that in FIG. 7, an exemplary case in which the intercooler 22 isdisposed in the refrigeration cycle apparatus 1 is shown.

By cooling a high-temperature middle-pressure refrigerant that has beendischarged from the first compressor 2, the inclination of theisentropic line of this refrigerant in the Mollier chart becomes steep.That is, the power required for the second compressor 3 to compress therefrigerant can be reduced. Note that the connecting portion of thebypass 8 between the first compressor 2 and the second compressor 3 maybe on the upstream side of the intercooler 22 or the downstream side ofthe intercooler 22. In the former case, a sudden pressure rise of thedischarge pressure of the first compressor 2 until the expander 5 startsup can be suppressed. This effect may be achieved by replacing an on-offvalve 9 with a flow control valve and by controlling the opening degreeof the flow control valve.

Further, in Embodiment 1, the heat medium that exchanges heat with theradiator 4 and the evaporator 6 is air, but other heat mediums may beused. For example, the heat medium exchanging heat with the radiator 4may be water, and the refrigeration cycle apparatus according toEmbodiment 1 may be used for supplying hot water. Further, the heatmedium exchanging heat with the radiator 4 and the evaporator 6 may bewater or brine, and this heat medium may be conveyed to the conditionedspace to air-condition the conditioned space.

Furthermore, in Embodiment 1, carbon dioxide, which has zero ozonedepleting potential and has an outstandingly small global warmingpotential compared to chlorofluorocarbon, is used, but the type of therefrigerant is arbitrary. However, the operating efficiency (COP) of therefrigeration cycle apparatus that employs carbon dioxide is lowercompared to refrigeration cycle apparatuses that uses conventionalrefrigerants. Therefore, it is highly advantageous to employ theinvention to a refrigeration cycle apparatus that uses carbon dioxide.Note that when using a refrigerant that is not compressed into asupercritical state, the radiator 4 functions as a condenser.

Further, in Embodiment 1, although the expander 5 and the secondcompressor 3 is mechanically connected (with shaft 7), the expander 5and the second compressor 3 may be connected electrically. For example,the expander 5 may be connected to a power generator, and the powerrecovered by the expander 5 may be converted into electric power that issupplied to the second compressor 3.

Furthermore, in Embodiment 1, although the refrigeration cycle apparatus1 (refrigeration cycle apparatus 51) determined whether steady operationis capable by using the temperature sensor 21, a pressure sensor may beused to determine whether steady operation is capable or not. Morespecifically, a pressure sensor may be disposed on both the dischargeside and the suction side of the second compressor 3. Additionally, whenthe detection values of these pressure sensors are above or equal to acertain threshold value, the refrigeration cycle apparatus 1(refrigeration cycle apparatus 51) may determine that steady operationis possible.

Embodiment 2

The invention can be embodied not only in the refrigeration cycleapparatus illustrated in Embodiment 1, but can be embodied in arefrigeration cycle apparatus with a configuration as below, forexample. Note that unless otherwise stated, Embodiment 2 is the same asEmbodiment 1.

FIG. 8 is a refrigerant circuit diagram of a refrigeration cycleapparatus of Embodiment 2 of the invention. A refrigeration cycleapparatus 52 according to Embodiment 2 is different with therefrigeration cycle apparatus 1 according to Embodiment 1 in thefollowing points. Other configurations of the refrigeration cycleapparatus 52 is the same as that of the refrigeration cycle apparatus 1.

First, the location of the first compressor 2 and the second compressor3 are opposite. Further, a check valve 13 is provided in place of thecheck valve 10. Furthermore, a bypass 11 and an on-off valve 12 areprovided replacing the bypass 8 and the on-off valve 9.

The check valve 13 is disposed between an expander 5 and an evaporator6, and regulates the refrigerant from flowing from the evaporator 5 tothe expander 5.

One end of the bypass 11 is connected between the second compressor 3and the first compressor 2, and the other end is connected between theexpander 5 and the check valve 13. This bypass 11 is provided with theon-off valve 12 that closes and opens the bypass 11.

Description of Operation

Description of the operation of the refrigeration cycle apparatus 52configured as above will be made. First, the operation of therefrigeration cycle apparatus 52 during steady operation will bedescribed. Then, the operation of the refrigeration cycle apparatus 52during start up will be described.

(Operation During Steady Operation)

The operation of the refrigeration cycle apparatus 52 during the steadyoperation will be described.

FIG. 9 is a refrigerant circuit diagram showing a refrigerant flowduring a steady state of the refrigeration cycle apparatus of Embodiment2 of the invention. During the steady state, the on-off valve 12 is in aclosed state. That is, in the steady state, the refrigerant is notallowed to flow in the bypass 11. Note that in FIG. 9, piping in whichthe refrigerant flows is depicted with thick lines.

The refrigerant that has been compressed into a high-temperaturemiddle-pressure refrigerant in the second compressor 3 is dischargedfrom the second compressor 3. This high-temperature middle-pressurerefrigerant is compressed in the first compressor 2 into ahigh-temperature high-pressure state (supercritical state), and flowsinto a radiator 4. The refrigerant that has flowed into the radiator 4transfers heat to the air sent by a fan 4 a and turns into alow-temperature high-pressure refrigerant. This low-temperaturehigh-pressure refrigerant flows into the expander 5. The refrigerantthat has flowed into the expander 5 is decompressed into a low-pressurerefrigerant with low dryness. During this decompression process, theexpander 5 recovers power. Then, the recovered power is supplied to thesecond compressor 3 through the shaft 7. The low-pressure refrigerantwith low dryness that has flowed out from the expander 5 flows into theevaporator 6 through the check valve 13. The refrigerant that has flowedinto the evaporator 6 receives heat from the air sent from a fan 6 a andturns into a low-pressure refrigerant with high dryness or alow-pressure super-heated gas refrigerant. The refrigerant that hasflowed out of the evaporator 6 is sucked into the second compressor 3.

Since the power recovered by the expander 5 is used as compression powerin the second compressor 3, the power required in the first compressoris reduced by the amount of power recovered. Hence, the refrigerationcycle apparatus 52 achieves energy saving.

(Operation During Start Up)

Next, the operation of the refrigeration cycle apparatus 1 during startup will be described.

FIG. 10 is a refrigerant circuit diagram showing a refrigerant flowduring the start up of the refrigeration cycle apparatus of Embodiment 2of the invention. During the start up, the on-off valve 12 is in anopened state. That is, in the start up, the refrigerant is allowed toflow in the bypass 11. Further, the fan 4 a that is sending air to theradiator is stopped or has a speed of rotation (rotation speed) lowerthan that of a steady state. Note that in FIG. 10, piping in which therefrigerant flows is depicted with thick lines.

The refrigerant that has been condensed in the first compressor 2 passesthrough the radiator 4 and reaches the expander 5. Further, with thestart up of the first compressor 2, the refrigerant on the outlet sideof the expander 5 passes through the bypass 11 and is sucked into thefirst compressor 2. Here, the check valve 13 prevents the refrigerant inthe suction side of the second compressor 3 to be sucked into the firstcompressor 2. That is, during the start up in which the secondcompressor 3 is suspended, the pressure on the suction side of thesecond compressor 3 becomes higher than the pressure on the dischargeside of the second compressor 3.

Note that during the state in which the second compressor 3 issuspended, even if the check valve 13 is not provided, the pressure onthe suction side of the second compressor 3 is higher than the pressureon the discharge side of the second compressor 3. The time for thesecond compressor 3 to start up after the first compressor 2 has startedup is a few seconds (with the refrigeration cycle apparatus 52 ofEmbodiment 2, about two to three seconds, for example). Accordingly,most of the refrigerant that is sucked into the suction side of thesecond compressor 3 is the refrigerant stored in the evaporator 6 (theevaporator 6 serving as a buffer), and therefore the pressure rise onthe suction side of the second compressor 3 becomes slack.

That is, the bypass 11 and the on-off valve 12 are the pressureregulating device of the invention. In Embodiment 2, in order toreliably obtain the pressure difference between the suction side of thesecond compressor 3 and the pressure of the discharge side thereof, thecheck valve 13 is provided.

That is, when the expander 5 is in a suspended state, the pressure onthe discharge side of the expander 5 becomes smaller than the inlet sideof the expander 5. Further, since the refrigerant flowing into the inletside of the expander 5 exchanges small amount of heat in the radiator 4,the refrigerant is low in density. That is, the controller 100 thatcontrols the bypass 11 and the on-off valve 12, and the rotation speedof the fan 4 a is the expander startup facilitating device of theinvention. Note that the check valve 13 does not have to be aconstitution of the expander startup facilitating device.

When the pressure difference of the expander 5 becomes large, theexpander starts up (driving starts). At this time, the pressure in theexpansion chamber of the expander 5 is as shown in FIG. 4 (same asEmbodiment 1). Since the pressure difference of the expander 5 duringthe start up is smaller than the pressure difference of the expander 5during the steady state, the refrigerant is slightly over-expanded, butpower (positive power) corresponding to “area D—area E” can be obtained.Thus, the driving of the expander 5 can be continued.

Meanwhile, the pressure of the compression chamber of the secondcompressor 3 that is connected to the expander 5 via the shaft 7 changesas shown in FIG. 5 (same as Embodiment 1). Since the pressure on thesuction side of the second compressor 3 is larger than the pressure onthe discharge side thereof (the pressure is inverse), it issupercompressed. The compression power at this time is the powercorresponding to the “area A—area B”, and is smaller than that of theconventional refrigeration cycle apparatus that equalizes the pressureon the discharge side and the suction side of the compressor (PatentLiterature 1, for example). Accordingly, it is easier to start up thesecond compressor 3 than the conventional refrigeration cycle apparatus.Further, depending on the extent of the inverse pressure, a powerrecovery corresponding to area B—area A can be obtained. The power inproportion to this will contribute to the stable start up of the secondcompressor 3.

Once the expander 5 and the second compressor 3 is started up, it ispossible to continue the driving of the expander 5 and the secondcompressor 3 even when the on-off valve 12 is closed. However, inEmbodiment 2, in order to reliably continue the driving of the expander5 and the second compressor 3, the on-off valve 12 is in an opened stateuntil the refrigeration cycle apparatus 52 is capable of operating inthe steady state.

More specifically, the controller 100 controls the on-off valve 12 asbelow.

When the second compressor 3 is driven continuously, the refrigeranttemperature discharged from the second compressor 3 rises. Additionally,the pressure on the discharge side of the second compressor 3 becomeslarger or equal to the pressure on the suction side. That is, it ispossible to operate the refrigeration cycle apparatus 52 in the steadystate.

In the refrigeration cycle apparatus 52, the temperature of therefrigerant discharged from the second compressor 3 is detected with atemperature sensor 21. In addition, the controller 100 determines thatthe refrigeration cycle apparatus 1 is capable of operating in thesteady state when the detection temperature of the temperature sensor 21is above or equal to a certain threshold value, and closes the on-offvalve 12. Furthermore, the rotation speed of the fan 4 a is changed tothe rotation speed for the steady state. A pressure sensor may be usedto determine whether the refrigeration cycle apparatus 52 is capable ofthe steady operation or not.

Note that even if a delay occurs in determining that the refrigerationcycle apparatus 52 is capable of operating in the steady state, becausethe check valve 13 is provided, the refrigerant flows to the expander 5without the suction pressure of the second compressor 3 droppingsuddenly. Accordingly, it is possible to reliably start up therefrigeration cycle apparatus 52 without the operation of a protectivedevice for low pressure and low temperature.

As described above, in the above-configured refrigeration cycleapparatus 52, the pressure on the suction side of the second compressor3 is made to be larger than the pressure on the discharge side thereofat least until the second compressor 3 is started up. Further, at leastuntil the expander 5 starts up, the pressure on the outlet side of theexpander 5 is made to be smaller than the pressure on the inlet side ofthe expander 5, and the refrigerant flowing to the inlet side of theexpander 5 is made to be low in density. Accordingly, it is possible tostart up the second compressor 3 and the expander 5 more reliably thanthe conventional refrigeration cycle apparatus.

Note that, it goes without saying that by merely making the pressure onthe inlet side of the second compressor 3 to be larger than the pressureon the outlet side of the second compressor 3, the second compressor 3and the expander 5 can be started up more reliably compared toconventional refrigeration cycle apparatuses. Further, it goes withoutsaying that by merely making the pressure on the outlet side of theexpander 5 to be smaller than the pressure on the inlet side of theexpander 5, the second compressor 3 and the expander 5 can be started upmore reliably compared to conventional refrigeration cycle apparatuses.

REFERENCE SIGNS LIST

1 refrigeration cycle apparatus; 2 first compressor; 3 secondcompressor; 4 radiator; 4 a fan; 5 expander; 6 evaporator; 6 a fan; 7shaft; 8 bypass; 9 on-off valve; 10 check valve; 11 bypass; 12 on-offvalve; 13 check valve; 14 four-way valve; 15 four-way valve; 21temperature sensor; 22 intercooler; 51 refrigeration cycle apparatus; 52refrigeration cycle apparatus; 100 controller.

1. A refrigeration cycle apparatus, comprising: a refrigerant circuitconnecting a first compressor, a first heat exchanger that serves as aradiator or a condenser, an expander, and a second heat exchanger thatserves as an evaporator in series with a piping; and a second compressorthat is driven by power recovered by the expander; a bypass bypassingeither between a suction side of the second compressor and a suctionside of the expander or between a discharge side of the secondcompressor and a discharge side of the expander; and an on-off valveprovided in the bypass, wherein the second compressor is a positivedisplacement compressor, the second compressor is connected to the firstcompressor in series, having one end of the bypass on its connectionline, and a pressure on a discharge side of the second compressor islower than a pressure on a suction side of the second compressor withkeeping the on-off valve as an opened state until the second compressoris started up.
 2. The refrigeration cycle apparatus of claim 1, whereinthe second compressor is disposed between the first compressor and thefirst heat exchanger.
 3. The refrigeration cycle apparatus of claim 2,further comprising a check valve provided at a position, which is closerto the first heat exchanger than the connection point of the bypass, ona passage between the first heat exchanger and the expander, the checkvalve regulating a flow of the refrigerant to the first heat exchanger.4. The refrigeration cycle apparatus of claim 1, wherein the secondcompressor is disposed between the second heat exchanger and the firstcompressor.
 5. The refrigeration cycle apparatus of claim 4, furthercomprising a check valve provided at a position, which is closer to thesecond heat exchanger than the connection point of the bypass, on apassage between the second heat exchanger and the expander, the checkvalve regulating a flow of the refrigerant to the expander. 6-9.(canceled)
 10. The refrigeration cycle apparatus of claim 1, furthercomprising: a heat medium sending device that sends a heat medium, whichexchanges heat with the refrigerant flowing in the first heat exchanger,to the first heat exchanger, wherein at least until the secondcompressor is started up, the rotation speed of the heat medium sendingdevice is reduced under the target rotation speed or the heat mediumsending device is stopped.
 11. The refrigeration cycle apparatus ofclaim 1, wherein the refrigerant flowing in the refrigerant circuit iscarbon dioxide.